This invention relates generally to improved hygrometer apparatus for measuring atmospheric water vapor content. The invention is particularly concerned with differential constant temperature sorption hygrometer transducer apparatus and constant temperature operation for the apparatus that have sensitive and stable performance, and which may be fabricated simply and economically. The invention also describes techniques for implementing its teachings in alternative transducer structures for application in a contained space or in free space.
The art of electric hygrometry is a mature and well developed art. Many, if not most, materials sorb and desorb water vapor as the ambient relative humidity increases or decreases. Associated with this sorption, there is usually a corresponding change in one or more properties of the material. To be useful in a hygrometer, the material used should have a reversible and reproducible humidity-property characteristic. Properties of air have been used to determine humidity and, in particular, a differential heat loss measurement of thermal conductivity as an indication of humidity is taught by U.S. Pat. No. 1,855,774. The invention disclosed apparatus that may be said to be the electrical counterpart to a wet bulb-dry bulb thermometer pair. A differential measurement is made by a heated resistor pair, one resistor in a reference or control volume, and the second exposed to the surrounding ambient environment.
The use of metal oxides in moisture sensing, and aluminum oxide in particular, is taught by U.S. Pat. No. 2,237,006 which describes a capacitance type moisture sensor using aluminum oxide as a hygroscopic layer between capacitor plates. U.S. Pat. No. 3,075,385 further develops the approach by using aluminum oxide as a dielectric in a capacitance hygrometer for radiosondes. U.S. Pat. Nos. 3,523,244 and 4,143,177 also describe capacitance hygrometers which use aluminum oxide as a moisture sensitive element between capacitor plates and the latter patent teaches us the use of aluminum oxide and silicon dioxide as moisture responsive elements in semiconductor device constructions where a closed volume measurement of water vapor content is desired.
Prior art hygrometer sensors are generally operated at ambient temperature and can easily become loaded with moisture which limits their response. Wet air-dry air reference system are cumbersome and are difficult to use in field operations. Aluminum oxide humidity sensors often demonstrate poor calibration stability owing to ambient temperature operation. The reaction of aluminum oxide with water, as in the aluminum oxide-hydroxide reaction rate, manifests itself as calibration instability, often confused with hysteresis. Many of these same deficiencies are exhibited by capacitive hygrometer transducers which use hygroscopic film materials as a dielectric. In particular, if such transducers become wet or saturated, an extremely long time must pass until they dry off and again become responsive to atmospheric moisture change.